1. Field of the Invention
The present invention relates to a piezo actuator driving circuit, and more specifically, to a piezo actuator driving circuit which not only can prevent signal distortion caused by process variation or temperature change occurring at the time of manufacturing a piezo actuator, but also can maintain a driving voltage so as to prevent a piezo actuator from malfunctioning or stopping, by adding a driving voltage maintaining section transmitting a current pulse to a piezo actuator.
2. Description of the Related Art
The current mobile phone market rapidly grows more than 20% every year. Recently, it is required to develop a mobile phone having various functions added thereto as well as a function of transmitting voice.
Accordingly, such functions as a camera, PDA, MP3 player, media player and the like are added, and mobile phones corresponding to 40% of the overall mobile phones support a camera function. Current mobile phones mainly adopt a 350 thousand pixel camera, but more and more mobile phones have a mega pixel camera mounted thereon. Recently, as 5 mega pixel and 7 mega pixel cameras are consecutively launched, a function of a camera module for mobile phone is also being enhanced. Particularly, as the direct competition with a digital camera is expected, an automatic focus function and automatic optical zoom function are required. In order to meet such requirements, it is urgent to implement a lens driving actuator and driver LSI, which have low power consumption and are small in size. Recently, such a technique as a lens driving method using a piezo element attracts attention.
In the driving method using a piezo element, noise and vibration does not occur at all and power consumption can be significantly reduced to ⅓, in comparison with a conventional driving method using a motor.
In order to drive such a piezo element, an accurate resonant frequency should be set, and a constant amplitude of driving voltage should be maintained. In general, however, the load capacitance of a piezo element is as large as hundreds of pF and process variation is also large, which can cause signal distortion and malfunction of an actuator.
FIG. 1 is a circuit diagram showing a piezo actuator driving circuit 100 according to the related art. As shown in FIG. 1, the conventional piezo actuator driving circuit 100 is composed of a pulse generator 100 which generates a voltage pulse VPULSE with a constant period, a first driver stage 102 which receives the voltage pulse VPULSE of the pulse generator 101 and buffers the voltage pulse VPULSE to output, a second driver stage 103 which converts the voltage pulse VPULSE, buffered and output in the first driver stage 102, into a current pulse to output, and a piezo actuator 104 which is connected to the second driver stage 103 so as to be charged and discharged by the second driver stage 103 and is driven by a constant amplitude of driving voltage pulse VACT.
Here, the first driver stage 102, which is composed of a buffering inverter, serves to buffer the voltage pulse VPULSE generated by the pulse generator 101.
The second driver stage 103 is composed of a first current source 103a to which a current pulse is transmitted to charge the piezo actuator 104, a first switching element 103b which receives the voltage pulse VPULSE of the pulse generator 101 and is connected to a power supply voltage VDD and the first current source 103a, a second current source 103c to which a current pulse is transmitted to discharge the piezo actuator 104, and a second switching element 103d which receives the voltage pulse VPULSE of the pulse generator 101 and is connected to a ground voltage and the second current source 103c. 
Here, the first switching element 103b is a PMOS transistor, and the second switching element 103d is an NMOS transistor. The first and second switching elements 103b and 103d are turned on or off according to the voltage pulse VPULSE applied by the pulse generator 101 so as to generate a current pulse for driving the piezo actuator 104.
The piezo actuator 104 can be modeled by a resistance, an inductor, and a capacitor element. The piezo actuator 104 includes an inductor stage composed of an inductor 104a, a first capacitor 104b, and a resistance 104c, which are connected in series; and a second capacitor which is connected in parallel to the inductor stage so as to resonate and is charged and discharged by the current pulse generated by the second driver stage 103 so as to maintain a constant amplitude of driving voltage pulse VACT.
The conventional piezo actuator driving circuit 100 shown in FIG. 1 operates as follows.
First, if the pulse generator 101 generates a voltage pulse VPULSE with the resonant frequency of the piezo actuator, the generated voltage pulse VPULSE is buffered through the first driver stage 102 so as to be transmitted to the second driver stage 103.
When the voltage pulse VPULSE generated by the pulse generator 101 is low, the first switching element 103b of the second driver stage 103 is turned on to transmit a current signal to the piezo actuator 104. Then, the second capacitor 104d of the piezo actuator 104 is charged to increase a voltage VACT which can drive the piezo actuator 104.
On the contrary, when the voltage pulse VPULSE generated by the pulse generator 101 is high, the second switching element 103d of the second driver stage 103 is turned on to transmit a current signal to the piezo actuator 104. Then, the second capacitor 104d of the piezo actuator 104 is discharged to decrease a voltage VACT which can drive the piezo actuator 104.
However, in the conventional piezo actuator driving circuit, the piezo element used in a conventional camera module has large variation in resistance, inductor, and capacitor at the time of mass production, and a change due to the temperature is also large. Therefore, the load impedance, or specifically, the load capacitance easily changes.
FIG. 2 is a diagram showing an output signal waveform of the conventional piezo actuator driving circuit. As shown in FIG. 2, when the load capacitance changes due to process variation or temperature change, the resonant frequency of the piezo actuator also changes. Therefore, a signal waveform desired by a user cannot be obtained with a current signal generated at the initially-set resonant frequency.
As an amount of current generated at the initially-set resonant frequency becomes insufficient due to process variation or temperature change, the driving voltage VACT decreases (to 7.5V), as shown in FIG. 2. Then, the piezo actuator cannot be driven, or can malfunction.